Fix npm package artifacts broken since migration to Hardhat

[frangio]

The npm package contains precompiled build artifacts for the contracts. These were generated by Buidler in artifacts and copied to the Truffle-expected location in build/contracts by the prepack.sh script.

openzeppelin-contracts/scripts/prepack.sh

Lines 5 to 8 in ee6348a

	# cross platform `mkdir -p`
	node -e 'fs.mkdirSync("build/contracts", { recursive: true })'
	cp artifacts/*.json build/contracts

This is broken since the migration to Hardhat which now creates a different directory structure in artifacts.

I'm considering we should just get rid of build/contracts from the npm package. Using those files doesn't really work consistently in Truffle. If we remove them there are many things that have to be adapted. Searching for "build/contracts" in the repo provides good pointers of where we should change something.

[frangio]

Fixed in #2503